Concrete prestressing apparatus (tank)



1959 J. w. HOWLETT 3, 0

coucnms PRES'IRESSING APPARATUS (TANK) Original Filed Aug. 16, 1963 3 Sheets-Sheet 1 Fig.4

Fig.5

INVENTOR. I

James W Howler? Aug. 12, 1969 J. w. HOWLETT CONCRETE FRESTRESSING APPARATUS (TANK) 5 Sheets-Sheet 2 Original Filed Aug. 16

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INVENTOR. James W Howletf 5 Sheets-Sheet 3 Aug. 12, 1069 J. w. HOWLETT CONCRETE PRBSTRESSING APPARATUS (TANK) Original Filed Aug. 16, 1963 nits Original application Aug. 16, 1963, Ser. No. 302,628, now

Patent No. 3,343,808, dated Sept. 26, 1967. Divided and this application Sept. 20, 1967, Ser. No. 669,002

Int. Cl. E04c 3/26, 5/08 US. Cl. 52-224 3 Claims ABSTRACT OF THE DISCLOSURE A cylindrical concrete tank is prestressed by a plurality of encircling reinforcing tendons wherein the tendons having opposed spaced ends are drawn together under high tension and firmly secured. To permit relative ease in drawing the tendon ends toward one another, yet maintain a tensioned condition after removal of the drawing force, wedge gripping couplers are employed which characteristically translate tendon tension or axial pull into one-way gripping forces, firmly engaging the tendon ends against axial pull out.

The invention relates generally to the art of prestressing various concrete structural members by the use of tensioning tendons such as wires, rods, stranded cables or bars, and more particularly to a prestressed concrete structure including secured tendons. The present application is a division of US. patent application Ser. No. 302,628, filed Aug. 16, 1963, now Patent No. 3,343,808 for Concrete Prestressing Method.

In various concrete prestressing operations, it is required that tension be applied to elongated tensioning tendons. This may involve either pretensioning or posttensioning of such tendons in relation to the concrete to be prestressed. Some tensioning techniques involve securing a rod at one of its ends to a stationary object, and then pulling the rod from its other end and securing it with the rod under tension. In reinforcing large cylindrical tanks or the like, it has been common to Wind wire or rods around the outer sidewall of the tank as a form of posttensioning, which creates the desired radially inward prestressing forces. A disadvantage of the wire winding technique is that the wires are relatively small, and therefore are subject to corrosion and breakage whereby maintenance becomes a significant problem. The use of steel rods in prestressing tanks has generally involved two end opposed axially aligned rods secured at their remote ends, with means being provided for drawing together the adjacent ends of the rods to place them under tension. Heretofore this has been accomplished by the use of turnbuckles or similar devices involving threaded couplings. Aside from the tedious task of tightening a turnbuckle, this type of tensioning device is not capable of achieving the same high order of prestressing that can be withstood by modern special alloy steel rods. For example, rods are available that have a tension strength in excess of 100,000 pounds per square inch, and which can withstand tension forces much greater than that provided in practice by turnbuckle devices. Moreover, it has been found impractical to use coupling devices wherein threads are formed in the tensioned rods, inasmuch as the threads create weakened shear planes incapable of withstanding high tensile forces of the magnitude above mentioned. Another technique has been to provide heads or enlargements on the confronting ends of the tendons. These heads are then engaged by plates which may be drawn together by a hydraulic jack, after which bolts are mounted through the plates in connecting fashion and nuts run down on the bolts to hold the plates against 3,460,300 Patented Aug. 12, 1969 separation upon subsequent release of the hydraulic jack. The plates and bolts thus form a holding frame or bracket for the tendons. While effective as a means for holding tendons under stress, the foregoing arrangement is relatively large and bulky, and considerably time consuming. Also, where center stressing of tendons is used in prestressed floor slabs, the foregoing arrangement may be too large and bulky for convenient use.

Briefly, the present invention involves a concrete structure engaged by at least one tendon having two adjacent ends drawn together under tension and securely connected to maintain a tensioned condition relative to the structure. The apparatus used includes improvements on the wedge grip couplers of the type disclosed in Howlett Patent No. 2,930,642, and in addition to providing a complete apparatus for connecting and tensioning opposed tendons.

Accordingly, it is a main object of the present invention to provide an improved prestressed concrete structure including tensioning rods placed under large tensile forces approaching their maximum tensile strength.

A further object is to provide an apparatus of the character described capable of prestressing circular tanks or the like by applying tension to rods disposed circumferentially around the tanks outer wall.

The invention possesses other objects and features of advantages, some of which of the foregoing will be set forth in the following description of the preferred form of the invention which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by the said drawings and description may be adopted within the scope of the invention as set forth in the claims.

Referring to said drawings:

FIGURE 1 is a side elevation of the concrete tank and apparatus of the present invention depicting post-tensioning encircling rods employed to prestress the tank.

FIGURE 2 is a plan view, in an enlarged scale and partially cut away and in cross-section, of tensioning apparatus.

FIGURE 3 is a fragmentary side view of a portion of the apparatus as suggested by the line 3-3 in FIGURE 2.

FIGURE 4 is a cross-sectional view taken substantially on the plane of line 44 of FIGURE 2.

FIGURE 5 is a cross-sectional view along plane 55 of FIGURE 3 of a one-way wedge grip coupler forming a part of the present invention, and operatively engaging two end opposed bar portions.

FIGURE 6 is a fragmentary cross-sectional view on enlarged scale of the wedge member and coupler housing showing the position of the parts during the inserting of the rod tendon.

FIGURE 7 is a view substantially the same as FIG- URE 6, but illustrating the coupler after the tendon has been placed under tension and is secured by the wedge member.

FIGURE 8 is a fragmentary plan view partially cut away and in cross-section, similar to FIGURE 2, but depicting tensioning apparatus forming an alternative embodiment of the invention.

FIGURE 9 is a fragmentary plan view similar to FIG- URES 2 and 8, depicting another alternative embodiment of the invention.

FIGURE 10 is a side view of a wedge grip nut shown in the embodiment of FIGURES 8 and 9.

FIGURE 11 is a cross-sectional transverse view taken along the planes suggested by the line 1111 in FIG- URE 10.

FIGURE 12 is a cross-sectional view taken along the axis of the bar member shown in FIGURE 10, and is suggested by the line 12-12 as shown in FIGURE 10.

Referring now to the drawings, the apparatus of the present invention includes means for drawing together and tensioning two generally aligned tedons 11 and 12 having opposed spaced ends 13 and 14, and includes a bar member 16; at least one coupler member 17 having gripping means 18 for axially receiving the bar member 16 and holding it against return movement; one of the members 16 and 17 being mounted on the tendon 11 in opposed confronting relation to the tendon 12; means for mounting the other of said members 16 and 17 on the tendon 11 in position for operative engagement with the member on the tendon 12; jack engaging means 19 and 21 provided respectively on each said tendon 11 and 12; and jacking means including jaws 23 and 24 for engaging the jack engaging means 19 and 21 for forcing the tendons 11 and 12 towards one another and operatively connecting the members 16 and 17 together, whereby the jacking means can be released to leave the tendons and members under tension.

In the embodiment of the invention depicted in FIG- URES 1-7, and in particular FIGURE 2, the means for mounting the bar 16 on the tendon 11 includes a second coupler 26 mounted on the tendon 11 and adapted to receive the end 27 of the bar member 16 in the same manner as does the coupler 17. The remote end shoulders of the couplers 17 and 26 are seen in this embodiment to provide the aforesaid jack engaging means 19 and 21, and the jaws 23 and 24 engage these shoulders to draw the tendons together.

As more specifically regards the structure of the couplers 17 and 26, reference is made to FIGURES 7 wherein a coupler 28 is illustrated and includes a housing 29 having an interior bore 31 for receipt of bar ends 32 and 33 to be connected. A sleeve-like wedge member 34 comprises the aforesaid gripping means 18 and is dimensioned to fit around the end 33, with an internal diameter having an interference fit therewith. The wedge 34 has an inner surface 36 formed with teeth 35 for biting into and gripping the bar 33, and has a single longitudinal slit 37 to enable radial expansiOn and compression against the bar. A plurality of longitudinally spaced annular inclined and interfitting cam planes 38 and 39 are formed respectively on the wall of the bore 31 and the external wall of the wedge 34, whereby axial pull on the bar is translated into radially internal gripping forces of the wedge teeth on the bar. In this regard, the interference fit between the bar and inner surface 36 provides an initial grip to start the cam action. The inclined and interfitting cam planes 38 and 39 are preferably formed as mating spiral threads, with the sleeve-like wedge member 34 thus being adapted to be threaded longitudinally in and out of the bore 31. To accommodate the bar 32, another wedge 40, similar to the wedge 34, is mounted in generally the same manner at the other end of the bore 31.

As an important feature of the present construction, and as above noted, an interference fit is provided between the respective bar and wedge members. More specifically, the external diameter of bar ends 32 and 33 is slightly greater than the normal unstressed internal diameter of the wedge sleeves 34 and 40 so that the axial insertion of the bar ends cause a resilient expansion of the wedge sleeves and a compressive gripping of the bar p-eripheries by the teeth 35 provided on the internal surfaces 36 of the wedge sleeves. To provide for such resilient expansion of the wedge sleeves 34 and 40, the normal unstressed external diameter of the wedge sleeves at cam planes 39 is less than the internal diameter of mating cam planes 38 provided on the internal wall of the housing. There is thus provided an important radial clearance between the wedge sleeve and the housing when the bar end, such as end 33 of FIGURE 6, is inserted axially into the wedge sleeve as depicted by arrow 40. The interference fit between the bar end and wedge sleeve, as aforementioned, causes wedge sleeve 34 to be displaced internally, as depicted by arrow 40a, by the movement of the bar end 33 until the shoulders 45 at the axial ends 4 of the spiral segments defining the mating cam planes 38 and 39 move into abutment as illustrated in FIGURE 6. In this position of the parts a small radial clearance between the wedge sleeve and the housing will still exist to provide manufacturing tolerance and proper functioning of the device. When return tension is placed on bar 33, however, the bar moves in a reverse direction as depicted by arrow 40b in FIGURE 7; and this return movement of the bar carries with it the surrounding wedge sleeve 34, as depicted by arrow 400. The latter m vement of the wedge sleeve closes the clearance between the mating cam planes and the wedging, camming action of the wedge sleeve against the bar 33 sinking teeth 35 into the periphery of the bar and securely gripping the latter with great holding force.

The method of the present invention in broad terms consists in the steps of mounting the coupler member 17 on the tendon 12 and providing the bar member 16 n the end of the tendon 11, which in the embodiment depicted in FIGURE 2, involves the provision of a coupler 26 secured to the tendon 11 and adapted to grippingly receive the end 27 of the bar 16. Force is applied by the jacking means to urge the tendons together and to bring the couplers 17 and 26 into gripping connecting engagement with the bar 16. In the embodiment depicted in FIGURE 2, the bar 16 is initially placed between the couplers 17 and 26 in spaced relation therewith, after which the couplers are drawn together into engagement with the respective ends of the bar. The force of the jacking means is then slowly released thereby enabling the bar to be gripped securely by the coupler wedges, It is noted that the tension initially provided by the jacking means is diminished after the force is released and as the teeth of the wedges bite into the bar. More specifically, as the teeth bite radially into the bar, there is a longitudinal displacement of the wedge relative to the coupler bore, which occurs primarily from the relative movement of the aforesaid interfitting cam planes of the wedge and housing bore. This relative movement is indicated by comparing FIGURE 6, showing the coupler as the bar is being inserted, and FIGURE 7, showing the coupler when the bar is under tension, it being observed that the bar and Wedge are moved to the left relative to their positions in FIGURE 6.

In order to achieve a maximum tension in the rods, it is frequently desirable to remove the slack created by the aforesaid setting of the wedges. In the embodiment depicted in FIGURE 2 this is accomplished by applying force to the couplers by the jacking means a second time, whereupon the tensile forces in the bar 16 are substantially eliminated, leaving it in an unstressed slack condition. By substantially taking up the slack in the bar 16, the force of the jacking means again can be released to leave the rods at the desired tension. With regard to the step of taking up the slack in the bar 16, it will be recalled that the wedge of each coupler has its cam surfaces formed into a spiral thread configuration, whereby rotation of the sleeve member moves it longitudinally relative to the coupler housing. By providing the sleeve members of the two couplers 17 and 26 with oppositely wound threads, the bar 16 when in slack condition can be rotated about its axis in a direction pulling the opposed bar ends tightly into the two couplers. Then, after the force of the jaws is released there is virtually no further longitudinal movement of the interfitting cam planes of the couplers, whereby the rods are maintained at substantially the tension achieved by the second application of force. While the rotation of the bar 16 is similar to taking up slack with a turnbuckle, there is no axial tension force on the bar as the slack is taken up. Rather, the rods are maintained in a tensioned condition by the jaws of the jacking means.

It will be appreciated that the couplers 17 and 26 can be provided on tendons of varying types, such as stranded cables, rods and the like. As shown in the embodiment of FIGURE 2, these couplers each are of the doubleended type described more fully with regard to FIGURE 5, and thus are adapted to engage a rod tendon in the manner described. It will be appreciated, however, that single-ended couplers could be used by welding or otherwise securing them to the tendons, be it a rod, cable or other configuration.

With more detail now regarding the aforesaid jacking means, there are provided a pair of arms 41 and 42 mounted on a link member 43 for pivotal movement relative to one another and having free ends 44 and 46. The aforesaid jaws 23 and 24 are mounted respectively on the ends 44 and 46, and each have bearing surfaces 47 and 48 for engaging the corresponding couplers 17 and 26 in the apparatus of FIGURE 2. Fluid pressure means 49 is connected to draw the arms together whereby the couplers 17 and 26 can be moved into engagement with the connecting bar 16 as previously described. The fluid pressure means 49, as best seen in FIGURE 2, is of generally conventional construction and consists of a housing 50 providing an interior cylinder 51 within which is mounted for reciprocation a piston 52. One end 53 of the housing is attached by a bracket 54 to arm 41, while the piston 52 is connected to the other arm 42 by a rod 55 which here projects axially through the housing 50 and piston 52 so as to project from a piston shaft sections 56 which extends through the opposite end 57 of the cylinder housing. A wedge grip nut 58, to be described more fully hereinafter, forms a protuberance on the outer end 59 of the rod 55 so as to bear against the adjacent end of piston shaft section 56. The opposite end 61) of the rod 55 may be similarly connected to the arm 42 by a wedge grip nut 61 which is arranged to bear against a flange 62 provided on a mounting bracket 63 fixed to the arm 42. Preferably, the mounting brackets 54 and 63 are of U-shape and are pivotally secured to arms 41 and 42 as illustrated. Housing 50 is here provided with inlet and outlet fittings 64 and 65 located adjacent the opposite ends 53 and 57 of the cylinder for conveying fluid under pressure into and out of the cylinder. As will be observed, the introduction of fluid under pressure into inlet fitting 64 will cause a displacement of piston 52 to the left, as seen in FIGURE 2, thus effecting a closing of the arms 41 and 42 and jaws 23 and 24. Contrariwise, the admission of fluid into the cylinder by way of fitting 65 and the withdrawal of fluid by way of fitting 64 will permit a displacement of piston 52 to the right, as seen in the drawing, and an opening of the arms and jaws. It will be noted that the rod 55 is disposed intermediate the link 43 and the free ends of the arms 41 and 42, and is substantially parallel to the link 43. The rod 55 is seen to be movable relative to the arm 41.

In further describing the arms and jaws of the jack ing means, it will be appreciated that the two arms are symmetrical and substantially identical to one another. Thus, reference is made now only to the arm 41 and jaw 23 mounted thereon. In particular, it is preferred that the free end 44 of the arm 41 includes a pair of bifurcated elements 44a and 44b adapted to fit around the corresponding tendon 11, as is shown in detail in FIG URES 3 and 4. Similarly, the jaw 23 also includes a pair of bifurcated elements 23a and 2312 which fit around the tendon 11, and are in general alignment with the elements 44a and 445). As shown, the elements 23a and 23b are spaced closer together than the elements 44a and 44b, whereby the tendon 11 fits snugly into the jaw while being able to pass relatively freely into the opening of the arm. In order to assure a firm grip of the couplers by the jaws, each jaw is pivotally mounted on its respective arm whereby the jaw and coupler need not move relative to one another as the arms are drawn together. Furthermore, in order to withstand the high forces involved in tensioning rods with the instant apparatus, the jaw 23 and free end 44 respectively have complementary arcuate bearing surfaces 66 and 67. In this manner, pivotal movement of the jaw relative to the arm is allowed but while still providing a sufficient bearing surface between these two members to transmit high force from the arms through the jaws to the couplers and rods. Preferably, the surface 66 is convex and is complementary to the concave surface 67. A pair of strap-like members 68 and 69, secured rigidly to the elements 44a and 44b, pivotally support the jaw 23 by means of stub shafts 70 and 71, but because of the arcuate bearing surfaces these stub shafts need not support the full load transmitted to the tensioned rods.

As further regards the structure of the arms 41 and 42, it is preferred that these arms consist respectively of pairs of spaced arm members 72, 73 and 74, 76. Considering the arm 41, the spaced bifurcated elements 44a and 44b may be formed as integral extensions respectively of the members 72 and 73. The arm members are secured rigidly together by means of nuts 77, bolts 78, and spacers 79. The bar 55 extends through the spaces between the arm members, and similarly, the link 43 is pivotally secured between the arm members.

It will be seen that the arms 41 and 42 as shown mounted at spaced positions on the link 43 have their center lines in relatively parallel relationship. Movement of the arms towards one another thus requires a minimum of pivotal movement of the jaws 23 and 24 relative to their respective arms, thereby minimizing the friction involved in moving the arms together. Inasmuch as the tendons 11 and 12 to be joined are not always spaced the same distance apart, it is desirable that the length of the link 43 be adjustable to provide the aforesaid generally parallel relationship of the arms. As here shown, link 43 is composed of a tubular center section and a pair of removable end members 81 and 82 which are in turn pivotally connected to the adjacent ends of arms 41 and 42 by pivot bolts 83 and 84. Since the force on the link during operation of the devices is in compression, end members 81 and 82 are provided with flanges, as seen in FIGURE 2, for abutting the opposite ends of the tubular center section 80. Preferably, ring clamps 85 and 86 are mounted around the opposite ends of the tubular center section for squeezing down upon the inserted portions of end members 81 and 82 so as to hold the latter in place, the opposite ends of the tubular center section being slitted to permit compression by the ring clamps against the end members. Adjustment of the length of the link 43 may accordingly be conveniently effected by dismantling the center section 80 and end members 81 and 82 and replacing the center section with one of appropriate (longer or shorter) length.

An alternative jacking arrangement suitable for providing the desired parallelism of the jaws can be provided in lieu of the pivotable jaw arrangement just described. More specifically, with nonpivotable jaws formed On the free ends of the jack arms, there can be provided in place of the link 43 an adjustable link consisting of a conventional hydraulic piston and cylinder such as that included in the fluid pressure means 49 already described. In using such a link, force could be applied to the jaws to draw them together as described hereinabove, but in short increments, with the adjustable link being correspondingly shortened after each increment to maintain the arms in the desired parallel relation. The shortening of the link is accomplished in a simple manner by valve means for transferring fluid from one side of the piston to the other as is necessary.

An alternative embodiment of the invention is depicted in FIGURE 8, wherein the bar 16a is provided directly on the end of the tendon 11a. As shown, the bar 16a and tendon 11a are integrally formed as a single rod, although it will be appreciated that the bar 16a can also be secured to other types of tendons. The coupler 17a, as shown in FIGURE 8, is again secured to the tendon 12, and preferably comprises a double-ended coupler of the type shown in FIGURE 5. Separate jack engaging means 19a and 21a are here provided as protuberances on the tendons 11a and 120, independent of the shoulders of the coupler 17a. Preferably, the protuberances are formed as removable wedge grip nuts, similar to the nuts 58 and 61 aforementioned, to be described more fully. To connect the tendons 11a and 12a, the jacking means force the two tendons together and into engagement with the two ends of the coupler 17a, whereupon the wedges of the coupler grippingly engage the tendons and hold them against return movement. As described with the embodiment in FIGURE 2, when the force of the jacking means is re leased, the setting of the wedge cam planes causes the initial tension in the tendons to be decreased. However, again in this embodiment, it is possible to take up the slack caused by this setting by applying force again for a second time thereby placing the ends of the tendons intermediate the wedge nuts in a slack unstressed condition. By providing the two wedges of the coupler 17a with oppositely wound spiral threads, rotation of the coupler causes the ends of the tendons to be drawn together tightly. Then when the jacking force is released, the tendons will remain in substantially the desired tension condition.

A third alternative embodiment of the invention is depicted in FIGURE 9, where again the coupler 17b is provided on the end of the tendon 12b and the bar 1612 is provided on the end of the tendon 11b. Unlike the embodiment of FIGURE 8, however, the coupler 17 need not be of the double-ended type, although shown as such, and could for example be welded or otherwise secured to the tendon 12. The jack engaging means 21b here is provided by an end shoulder of the coupler 17b, while a removable wedge nut 19b mounted on the bar 16 spaced from its end preferably serves as the other jack engaging means. In drawing the tendons toward one another, the bar 16b is moved axially into gripping engagement with the coupler 17b, thus holding the tendons under tension. While it is appreciated that this embodiment of the invention does not afford the feature of rotational slack take-up as previously described regarding the other two embodiments, it nevertheless will be seen that a fewer number of parts are here required to join the opposed tendons than in the other embodiments. Since in various instances rotational slack take-up is n t necessary, the reduction of parts is of course advantageous. Wedge grip nuts 19a, 21a, 19b, 59 and 61 are all here shown of similar construction. One of these nuts, identified as 19a, is illustrated in detail in FIGURES 11 and 12. The nut is particularly constructed for mounting either permanently or temporarily upon a bar or rod and for this purpose is split longitudinally for removal from the rod in two half sections. Accordingly, the nut housing is formed of a pair of half-cylindrical housing sections 91 and 92, and these fit around a pair of halfcylindrical wedge sections 96 and 97, the latter cooperating to surround rod 12a in the same manner as wedge member 34. The wedge sections 96 and 97 have internal bar engaging surfaces 98 formed with teeth for biting into and compressively engaging and gripping the periphery of the bar. Also similar to the coupling construction above described, the wedge and housing sections are formed with a plurality of longitudinally spaced annular inclined interfitting cam planes formed on the inner surfaces 93 and 94 of the housing sections and the external walls 99 of the wedge sections so as to translate pull of the bar in an axial direction relative to the nut into radially gripping forces of the wedge sections 96 and 97 on the bar. The housing sections 91 and 92 are here secured together by a pair of threaded bolts 101 and 102 which extend through bores 103 in section 91 into threaded engagement with tapped bores 104 in housing section 92. It will be seen from the foregoing that the wedge nut can be readily mounted at any intermediate position on a bar member, thus enabling it for use with the fluid pressure means 49, and the embodiments of FIGURES 8 and 9 as described hereinabove. An important feature of the nut is that it can be easily 8 removed from the bar after it has been used. Consequently, the wedge nuts in the embodiments of FIGURES 8 and 9 can be taken off of the tendons after they have been connected, thus enabling their use elsewhere.

In FIGURE 1, the present invention, as embodied in the above described methods and apparatus, is generally depicted in the operation of tensioning a plurality of rods 106 around a circular tank 107. A bracket 108 is provided on the arm 41 of the jacking means to enable the jack to be supported by a cable (not shown) from a suitable crane or the like disposed on top of the tank 107 and adapted to move along the tanks periphery. In mounting the rods 106 around the tank, a plurality of rods may be simply spliced together at adjacent ends by a coupler 28, as shown in FIGURE 5. This generally is best accomplished by first mounting the wedges separately on the rod ends, after which the housing is threaded onto the pairs of opposed wedges. These spliced rods are then disposed around the tank with the spaced opposed ends of two rods remaining to be joined. As shown, a pair of couplers 17 and 26 and a connecting bar 16 are used to join the remaining ends in the manner described hereinabove with regard to the embodiment of FIGURE 2, thus placing each of the encircling rod assemblies under tension. A high pressure fluid compressor 109 is connected to the fluid pressure fittings 64 and 65 of the hydraulic actuator 59 by a pair of conduits 111 and 112. The compressor 109 is preferably portable for movement around the tank together with the jacking means. The compressor illustrated is one manufactured by Lincoln Engineering Company of St. Louis, Mo., and ordinarily sold as a high pressure lubricant pump. This unit is air-motor operated and may be produced to provide a 70 to 1 ratio so that pounds per square inch air pressure will yield 7,000 pounds per square inch oil pressure for application to the hydraulic actuator 39. Of course, any suitable commercially available high pressure hydraulic source may be used.

It will be appreciated that with a relatively large tank, the joining together of the couplers 17 and 26 may not provide the desired tension in the tendons around the whole tank, since nonuniformity of tension is likely to be caused by friction. Consequently, it may be desirable to draw each pair of opposed tendon ends together to tension them, rather than simply splice them as shown, in addition to applying tension to the last pair of ends to be connected. To do this, considering the embodiment of FIGURE 8, for example, the wedge members of the coupler would be separately mounted on the tendon ends, apart from the coupler housing, but just far enough to hold them on during initial tensioning. Then the coupler housing is threaded onto the wedges in the manner described to form the splice. By mounting the split wedge nuts on each tendon end, as shown in FIGURE 8, it is then possible to bring the jack to each splice joint and draw the tendons all the way into the wedges, thus taking up slack at each splice position. After all the splices are tightened in this manner, further removal of slack can be accomplished, as described above, with regard to the second application of force at each splice connection, and rotation of the coupler to draw up the tendon ends as tightly as possible. Each of the splice connections, of course, can similarly be made in accord with the embodiments of the invention depicted in FIGURES 2 and 9. In order to measure the tension in the bars to assure that the desired stressing conditions are achieved, it is preferable that a device such as a Howlett Extensometer be used in conjunction with each slack take-up point to observe the tensile forces as the jack is operated. Preferably, the housing of the present coupler is formed of high strength steel so as to carry the loads imposed upon it and the wedge members are preferably formed of hardened steel so as to take a proper bite into the rods.

From the foregoing it is apparent that the present invention affords a highly convenient method and apparatus for connecting and placing tendons under tension, and which can be utilized to utmost advantage in providing extremely high tensile forces that approach the maximum tensile strength of the tensioned members.

I claim: 1. A prestressed concrete structure comprising: a concrete body; at least one reinforcing tendon engaging said body to impart prestressing forces thereto and providing two opposed ends positioned in spaced confronting relation with a bar portion provided on one of said ends; and a coupler mounted on the other end and connecting said other end to said end having said bar portion to secure said ends relative to each other, said coupler being formed with an interior bore with a radially expansible and compressible sleeve-like wedge member mounted therein, said wedge member having inner bar engaging surfaces formed with teeth biting into and compressively engaging in gripping the periphery of said bar portion, said teeth being further formed and dimensioned to have an interference fit with said bar portion and, said coupler being further formed with a plurality of longitudinally spaced annular inclined and interfitting cam planes formed on the internal wall of said bore and the external wall of said wedge member whereby axial pull of said bar out of said bore is translated into radially internal gripping forces of said wedge member on said bar, said cam planes being formed and dimensioned to allow radial expansion of said wedge member upon insertion of said bar portion therein and radial contraction of said wedge member upon tensioning of said tendon; said tendon, coupler, and bar being under tensile stress. 2. A prestressed concrete structure as defined in claim 1 wherein said concrete body is formed as a concrete tank having a cylindrical concrete tank wall, said first opposed end is formed with a bar portion, and said coupler is provided with, at longitudinally opposed portions thereof, a complementary and opposing pair of said wedge members and cam planes each formed for interference fit with said bar portions and formed to allow radial expansion and compression of said wedge members, said coupler engaging said opposed bar portions of said tendon whereby tension on said tendon and bar portions is translated into radially internal gripping forces of said pair wedges on said bar portions, said tendons, coupler and bar portions being under tensile stress.

3. A prestressed concrete structure comprising:

a concrete body;

at least one reinforcing tendon engaging said body to impart prestressing forces thereto and providing two opposed ends positioned in generally aligned spaced confronting relation with a bar portion provided on each of said ends;

first and second couplers separately mounted on and secured to said bar portions, each of said couplers being formed with opposing interior bores having a radially expansible and compressible sleeve-like Wedge member mounted therein, said wedge members having interior bar engaging surfaces and the external walls of said wedges and the internal wall of said coupler being formed with a plurality of longitudinally spaced annular inclined and interfitting cam planes; and

a connecting bar mounted between said couplers with the ends thereof positioned in said bores and engaged by said surfaces, said engaging surfaces being further formed and dimensioned to have an interference fit with said connecting bar and said cam planes being formed and dimensioned to allow radial expansion of said wedge members upon insertion of said connecting bar and radial contraction of said wedge members upon tensioning of said tendon; and

said tendons, couplers and connecting bar being under tensile stress.

References Cited UNITED STATES PATENTS 1,366,732 1/1921 Hoyt 287- 1,854,140 4/1932 Hopkins 24-136 2,315,895 4/1943 Crom 52224 2,945,720 7/1960 Osmun 29486.17 2,930,642 3/ 1960 Howlett 287-1 14 3,343,808 9/1967 Howlett 287-114 FOREIGN PATENTS 155,756 3/1954 Australia. 864,170 3/196-1 Great Britain. 86,778 10/ 1920 Switzerland.

ALFRED C. PERHAM, Primary Examiner US. Cl. X.R. 

